Question(s) for those logging EGT with MegaSquirt
 

I'm wondering if someone can help me troubleshoot my EGT input/readout for my MS1.

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Here is the setup:

Spruce Micro1000 k-type EGT probe a couple inches from #4 cylinder on manifold

Chromel/Alumel k-type extension cable

DPDT on-on switch w/ one side going to...

ISSPRO egt gauge

and the other going to... a SymTech Labs MegaSquirt EGT Interface Module (which is the EGT circuit built and mounted on a small external circuit board instead of the proto area)

from the SymTech board it goes through my DB37 to JS4 on the v.3 board

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I know that extra connections within the chromel-alumel wiring can cause calibration errors, but the switch is essentially at the end of the run where it goes into the gauge's signal amplifier and the SymTech circuit, so I was thinking that it shouldn't be much of an issue? The switch is there to make it easier to use a single sensor for logging and then monitoring via gauge when I'm not logging.

The gauge itself seems to function properly and as expected. It reacts to engine conditions and gives readings that are essentially in the range that I would expect. But when I flip the switch to provide signal to the MS1, I get a reading of ~1300F that does not fluctuate with changes in exhaust temperature.

I'm running MSnSE hr_11d firmware and using TunerStudio, and everything was already set up to read/log EGT, so I didn't have to modify any INI files to get it working.

Before I go tearing things apart to inspect and troubleshoot, I thought I'd ask if anyone has run into behavior of this sort before, and if it sounds like there's something simple that I've screwed up or forgotten to do.

EDIT: I think I've got something screwed up inside the MS1. If I remove the EGT input wire from my subharness before it goes into the DB37, the EGT reading jumps to ~450F. So it sounds like I'm getting some sort of a voltage leak going into JS4, is that right?

Allow me to pick your brain. Does the output require a 12 or 5v pull-up resistor?

The SymTech circuit requires a 12v supply. 5 wires connected (just like the circuit described here).

1- 12v
2- ground
3- chromel
4- alumel
5- output to MS1

I'm using the original 1000C circuit.

I think I've discovered the problem. I seem to be getting some voltage bleeding over from the constant baro going into JS5. The only thing I can figure is that somehow I managed to get a tiny sliver of solder bridge on the back side of the board (below the main chip)?

I used a bright LED flashlight to peek under the chip, and I don't see any indication of a solder bridge, but admittedly it is difficult to get a good look. And I can't think of any other explanation.

Any other ideas? Is this fixable, and if so how do I go about it?